I am a dual boxer, and was trying to come up with a way on my "spam" macro to throw an occasional heal from my back toon to the front one. I ended up with a macro with two /castrandom statements, and a unique usage of ,,, to get the job done. Works pretty well for me, might work for you as well...

/assist focus
/targetenemy [noexists]
/castrandom [exists,harm,nodead] lightning bolt, earth shock, flame shock,lightning bolt,,,
/castrandom [target=focus] lesser healing wave,,
/startattack